Re: [PATCH] x86/cpu: identify uninitialized CPU data using BAD_APICID

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, May 04, 2026 at 01:31:34PM +0200, Jan Beulich wrote:
> On 30.04.2026 13:46, Roger Pau Monne wrote:
> > Uninitialized cpu_data[] entries have the apicid field set to BAD_APICID,
> > not boot_cpu_data.apicid.  Fix the check in cpu_smpboot_free() to use the
> > correct condition.
> > 
> > Fixes: 7126b7f806d5 ("x86/CPU: re-work populating of cpu_data[]")
> 
> I think this isn't correct, and the issue here is me having overlooked a
> dependency between that commit and the one introducing the line which
> you change: 8c15d3d18725 ("x86/SMP: guard socket_cpumask[] access in
> cpu_smpboot_free()"). Both changes were committed close together, but
> they were entirely separate submissions (well over a year apart). Hence
> as an individual patch that latter commit was correct, but on top of
> 7126b7f806d5 it would have needed adjustment. Since 7126b7f806d5 wasn't
> backported (while 8c15d3d18725 was), the fix here also shouldn't be
> backported to anything earlier than 4.21 (which indeed the Fixes: tag
> better expresses).

I see, right, in the git history 8c15d3d18725 is indeed later than
7126b7f806d5, and hence 7126b7f806d5 was correct when introduced.  The
bug in cpu_smpboot_free() was there, but 7126b7f806d5 made it no
worse.  It was 8c15d3d18725 that would indeed need to be adjusted to
pick up the new initialization value.

Thanks, Roger.